In general, a variety of optical films, such as a light diffusion plate, a light diffusion film, and a prism film, are used in flat-panel display devices, such as liquid crystal displays (LCDs). Recently, there is a need to improve the optical efficiency of combined optical films while providing various functions of optical films. Especially, there is a need to adjust lens patterns on the surface of optical films and in width-to-depth ratio (hereinafter “curvature”) of the lens patterns.
In order to produce a linear lens pattern, e.g., a lens pattern on a conventional prism film, a direct carving method has been described as follows: first, nickel or copper is plated onto a surface of an iron roll, and a linear prism lens pattern is formed through Bite processing, as shown in FIG. 1. However, it is difficult to produce a zero defect roll and only a linear lens pattern can be formed by the Bite processing. In addition, when two-axis processing is performed to make an angle of 90° using a multi-axis processor, a pyramid lens pattern can be produced; however, the processing time is very long (usually 15 to 30 days), thereby drastically increasing manufacturing cost.
Furthermore, as a method for producing a non-linear lens pattern, the micro lens film (MLF) manufacturing method has been described as follows: as shown in FIG. 2, spherical beads are applied on a polyethylene terephthalate (PET) film having ultraviolet- or heat-curable adhesive coated thereon, and then the adhesive is cured. Next, either nickel (Ni) is subjected to electroless plating on the film, or the PET film having the ultraviolet- or heat-curable adhesive coated thereon is laminated and cured, and the PET film is exfoliated to manufacture products with a mold, such as a belt-type mold. However, productivity is low because a seam inevitably forms when a belt-type mold is used in manufacture. Direct carving on a cylindrical roll is not possible. Product quality can be greatly affected by the shape and degree of dispersion of beads. Adjustments to the number density and shape of lenses by the application of beads and to the formation of various lens patterns and curvature thereof are limited.
Another manufacturing method has been described as follows: as shown in FIG. 3, a photomask having a lens pattern of an appropriate size is used to expose light on a PET film that has photoresist coated thereon. Next an exposed area or an un-exposed area of the photoresist is developed, and a raised lens pattern is formed through heat treatment. Either nickel (Ni) is subjected to electroless plating thereon, or the PET film having an ultraviolet- or heat-curable resin coated thereon is laminated and then cured, as in the method of bead application; and the PET film is exfoliated to manufacture products with a mold, such as a belt-type mold. However, this method also has the same disadvantages as the MLF manufacturing method in that various lens patterns and curvatures cannot be implemented because a seam inevitably forms when a belt-type mold is used. Direct carving on a cylindrical roll is not possible; the number density of the lens is too high, cost for making the mold is too high, and there is a limitation in forming lens.
Yet another manufacturing method has been described as follows: as shown in FIG. 4a, a photoresist layer is deposited onto a cylindrical roll having copper (Cu) plated thereon. The planar shape of a desired lens pattern is etched by laser processing: the area exposed to laser is developed in the case of a negative photoresist; and in the case of a positive photoresist, the area exposed to the laser decomposes. Then etching solution is applied. But, the cross-section of the resulting lens pattern is formed only in a shape such that the depth (P) of the lens pattern is less than the planar radius {(R+2P)/2} of the lens pattern, as shown in FIG. 4b. The curvature is limited by the depth of the resulting lens pattern because the etching speed parallel to the roll surface and the etching speed perpendicular to the roll surface (in the direction of depth) are theoretically identical. In other words, the diameter R of the laser processing should be zero to form a hemisphere with a depth of one half that of the planar diameter of the lens pattern (P/2P=½). However, corrosion speed at the center of diameter R during exposure to etching solution is statistically faster than other parts of the pattern, and corrosion speed slows as etching proceeds, thereby causing the speed in every direction to be virtually identical. In addition, the lens pattern may be made deeper by physically accelerating the injection speed of the etching solution, thereby producing a higher corrosion speed in the direction of depth; however, the technique is limited to a depth of one hemisphere in curvature. In another method, if the depth is made deeper by reducing the diameter during laser processing, more etching is needed. Accordingly, the copper plating lacks uniformity, and corrosion speed is irregular and more easily disrupted by foreign objects and the like, thereby causing a crushed peanut shape, the shape of snowman, or other irregular shapes from the collapsed boundaries between lens patterns.